1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrostatic copying machine, a laser printer, or the like in which a toner image is fixed to a fixing material.
2. Description of the Related Art
In a fixing device incorporated in a copying machine using an electrophotographic process, a developer which is toner supplied onto a fixing material is heated and melted to fix the toner to the fixing material. A method of using radiated heat based on a halogen lamp (a filament lamp) is widely used as a method for heating toner usable for a fixing device.
With respect to the method of using a halogen lamp as a heat source, a structure is widely used, i.e., paired rollers are provided such that a predetermined pressure can be applied to the fixing material and toner, at least one of the paired rollers is used as a hollow column, and a halogen lamp arranged like a column is arranged in the inner hollow space. In this structure, the roller provided with a halogen lamp forms an acting part (nip) at a position where the roller contacts the other roller, so that pressure and heat are applied to a fixing material and toner guided to the nip. That is, the fixing material, i.e., a paper sheet is passed through a fixing point which is a press contact part between a heat roller provided with a lamp and a press roller which rotates as a slave to the heat roller, and thus, toner on the paper sheet is melted and fixed to the paper sheet.
In the fixing device using a halogen lamp, light and heat from the halogen lamp is radiated in all directions to the entire circumference of the heat roller, so that the entire roller is heated. In this case, it is known that the thermal conversion efficiency is 60 to 70%, which is low, the power consumption is large, and the warm-up time is long, in consideration of the loss at the time when light is converted into heat, the efficiency at which air in the roller is warmed to transfer heat to the holler, and the like.
Hence, in recent years, as a heat source for a heat roller, an induction heating method has been practiced, in which a heat coil is provided inside a heat roller and a high-frequency current is supplied to the coil, so that heating is carried out by induction heating.
For example, Japanese Patent Application KOKAI Publication No. 59-33476 discloses a technique in which a roller having a thin metal layer on the outer circumference of a ceramic cylinder is comprised and an induction current is passed through the thin metal layer of the roller by the conductive coil to achieve heating.
Japanese Patent Application KOKAI Publication No. 78-76620 discloses a device in which a conductive film is heated by a magnetic field generation means, and toner is fixed to a paper sheet kept in tight contact with the conductive film. A heat belt (conductive film) is sandwiched between a member forming part of the magnetic field generation means and a heat roller, to form a nip.
Japanese Patent Application KOKAI Publication No. 258586 discloses a method which uses a heat generation member in which a coil is wound around a core provided along the rotation axis of a fixing roller, and which achieves heating by producing eddy currents through the fixing roller.
However, it is known that a temperature distribution appears on the surface of a heat coil, depending on the shape of the coil, in the case of electrically conducting a coil to heat a heat roller.
Hence, at the time when the roller surface reaches a predetermined temperature after a predetermined time has elapsed from the electric conduction to the coil, it is normally necessary to rotate the heat roller and a press kept in contact with each other, to uniform the temperature distribution of the heat roller.
However, the temperature of the heat roller which has once risen drops rapidly since the heat roller and the press roller are rotated. Therefore, a high-frequency current must be continuously supplied to heat the heat roller until the temperature rises up to a temperature which enables fixing.
This means in which the image forming operation is stopped until the fixing operation is enabled, results in a problem that the electric power required for heating increases.
The present invention has an object of providing an image forming apparatus capable of shortening the time from when the power source is turned on to when copying can be accepted, i.e., a so-called first copy time, and also capable of supplying an effective maximum electric power for a fixing device without exceeding the upper limit of power consumption.
The present invention provides a fixing device for use in an electrophotographic apparatus arranged in a structure in which a high-frequency current is flowed through an induction coil provided close to an endless member having a metal layer made of a conductive material and the endless member is caused to generate heat to heat and fix a material to be fixed, while maintaining a first temperature,
wherein when electric conduction to the coil is started and a start-up operation is started, the endless member is rotated when the metal layer reaches a second temperature higher than the first temperature, and for a predetermined time from the start of rotation of the endless member, a high-frequency current having a size corresponding to a control value for a third target temperature is supplied to the coil, the third temperature being lower than the second temperature and higher than the first temperature.
The present invention also provides a fixing device for use in an electrophotographic apparatus arranged in a structure in which an alternating current is passed through an induction coil provided close to an endless member having a metal layer made of a conductive material, and the endless member is caused to generate heat to heat a material to be fixed,
wherein in a warm-up period, an electric power amount supplied to the induction coil is changed in a plurality of steps, in compliance with the operation of another component.
The present invention further provides a fixing device for use in an electrophotographic apparatus arranged in a structure in which a high-frequency current is passed through an induction coil provided close to an endless member having a metal layer made of a conductive material, and the endless member is caused to generate heat to heat a material to be fixed, while maintaining a first temperature,
wherein when electric conduction to the coil is started and a start-up operation is started, the endless member is rotated when the metal layer reaches a second temperature higher than the first temperature.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.